Method for producing photographic emulsion coatings

ABSTRACT

A surface of an object is coated with a uniform thickness of a photographic emulsion. In the coating method, there is utilized an apparatus which comprises a smaller substantially enclosed chamber which communicates at its upper end with a vertical open larger chamber. Jet streams of liquid emulsion droplets and an inert gas are introduced into the smaller chamber. By elutriation, larger droplets fall to the bottom of the smaller chamber and droplets sufficiently small in accordance with Stokes law pass into the larger chamber. Here, those droplets which are small enough according to Stokes law for the larger chamber rise to leave the larger chamber from its open end and droplets having a size large enough to descend according to Stokes law settle on the surface of an object positioned within and at the base of the larger chamber to form the emulsion coating on the surface.

United States Patent Buckingham'et al. Mar. 7, 1972 [54] METHOD FOR PRODUCING 3,382,845 5/1968 Jester ..1 18/326 PHOTOGRAPHIC EMULSION COATINGS Primary ExaminerWilliam D. Martin Assistant Examiner-William R. Trenor Inventors: Robert kinglmm, Hawthorne; Attorneyl-lanifin and Jancin and lsidore Match Harold H. Herd, Ossining, both of NY. [73] Assignee: International Business Machines Corpora- [57] ABSTRACT tion, Armonk, NY. A surface of an object is coated with a uniform thickness of a photographic emulsion. In the coating method, there is util- [22] Flled' 1969 ized an apparatus which comprises a smaller substantially en- [2] 1 Appl. No.: 887,688 closed chamber which communicates at its upper end with a vertical open larger chamber. Jet streams of liquid emulsion droplets and an inert gas are introduced into the smaller [52] US. (,I ..l17/34, 1111780202? l1ll78//1 (5)166 chamben By elutriation larger droplets fall to the bottom of [51] Int Cl M 08 G03c1/74 1/28 the smaller chamber and droplets sufficiently small in ac- [58] Fie'ld l3 4 104 DIG cordance with Stokes law pass into the larger chamber. Here, 41 1 5 those droplets which are small enough according to Stokes law for the larger chamber rise to leave the larger chamber from its open end and droplets having a size large enough to [56] References Clted descend according to Stokes law settle on the surface of an ob- UNITED STATES PATENTS ject positioned within and at the base of the larger chamber to 2 649 358 8/1953 P I ln/DIG 6 form the emulsion coating on the surface.

a mer 2,736,289 2/1956 Allen ..1 18/326 5 Claims, 3 Drawing Figures 35 HOT WATER j I 33 ATTORNEY' PATENTEDIAR 71972 INVENTORS ROBERT L. BUCKINGHAM HAROLD H. HERD llH COARSE PARTICLES E L C T R A P CUT OFF DRAINS PARTICLE SIZE BY JIWWMM DISTRIBUTION CURVE CHART HNE PARTICLES IH' 'Hll II], II

F l G 2 FRACTION PARTICLES METHOD FOR PRODUCING PHOTOGRAPHIC EMULSION COATINGS BACKGROUND OF THE INVENTION This invention relates to coating methods and devices. More particularly, it relates to a method for coating photographic emulsions on a surface at any desired thickness with a uniform thickness to close tolerances, and device for carrying out this method.

With the advances occurring in the techniques for producing printed circuits and monolithic solid state circuitry, there has arisen the need for providing in these techniques photographic emulsions which are quite thin and which are substantially uniform in thickness throughout. Known coating methods such as the wired mandril, the spinning, the casting, etc., have proven to be unsatisfactory in this regard.

Accordingly, it is an important object of this invention to provide a method of coating surfaces with photographic emulsions of any desired thickness with a uniform thickness to close tolerances.

It is another object to provide a device for carrying out the methods as set forth in the preceding objects.

SUMMARY OF THE INVENTION Generally speaking and in accordance with the invention, there is provided a method of coating a surface with an emulsion. The method first comprises the step of positioning in and at the base of a first and larger vertical chamber which communicates with a second smaller and substantially enclosed chamber at the upper end of the second chamber, an object having the surface to be coated. There is then introduced into the second chamber liquid droplets of the emulsion under gas pressure while controlling the volume of the gas whereby, by elutriation, droplets of the emulsion exceeding a given size fall to the bottom of the second chamber and droplets sufficiently small according to Stokes law for the second chamber enter into the first chamber. Those droplets of the droplets entering the first chamber and small enough according to Stokes law for the first chamber leave the first chamber through its open end. Those droplets entering the first chamber and large enough to descend according to Stokes law for the first chamber settle down on the surface of the object to form the emulsion coating thereon.

Also, and in accordance with the invention, there is provided a device for coating a surface with a photographic emulsion which comprises a first larger vertical open chamber and a second smaller substantially enclosed chamber which communicates with the first chamber near the upper end of the second chamber. There are included means for introducing under a chosen gas pressure liquid droplets of the emulsion into the second chamber and means for positioning an object having the surface to be coated within and at the base of the first chamber.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. I is a view, partly in section, of an illustrative embodiment of a device for carrying out the method of the invention.

FIG. 1A is a section taken along lines 1A of FIG. 1 looking in the direction of the arrows; and

FIG. 2 is a particle size distribution curve chart.

DESCRIPTION OF A PREFERRED EMBODIMENT The device shown in FIGS. 1 and 1A is a spray coating apparatus which is employed to expose a plate to a spray or fog of droplets of uniform particle size, the droplets being permitted to settle on the plate to a predetermined thickness. The droplets may be classified by gas elutriation so that droplets in a control range of sizes are placed in a cloud or settling chamber in the device of FIGS. 1 and 2. The density of the fog, i.e., the number of droplets per unit volume, and the diameter of the ;roplets determines the length of time that the plate is in the chamber, such time determining the thickness of the final coating. In the operation of the method, provided that the droplet diameter is sufficiently smaller than the wet film thickness, the random spatial distribution of the droplets in the fog produces by settling, a uniform coat on the plate. The attainment of uniformity is aided by the enabling of the control of the temperature gradient at the surface of the plate whereby the droplets are permitted a controlled time to coalesce and to sufficiently flow to convert the small spheres to a uniform film prior to cooling, i.e., the solidifying of gelatin in a photographic emulsion.

Referring now to FIGS. 1 and 1A, there is provided therein an angled substantially vertical cylindrical column 10 which communicates with a larger vertical cylindrical settling chamber 12, column 10 being constituted to substantially surround chamber 10. A photographic emulsion container 17 which can contain a water based or oil based gelatin, for example, is maintained at a desired temperature by a heater l6 and its associated coil. The emulsion from container I4 is introduced into column 10 through a valve control 18 and a jet 20.

An inert gas is introduced into column 10 through a pressure regulator 22 and a nozzle 24. The volume of the inert gas, which suitably may be air, and which is entered into column 10 is controlled through pressure regulator 22 whereby the elutriation of large drops which would cause disturbances in the gas emulsion mixture are permitted to fall back to the bot tom of column 10 and to be removed therefrom via a suitable drain provided for such purpose. Droplets in column 10 which are smaller than the Stokes law in the column are carried up and out of the column into chamber 12 which may suitably be termed a fog or settling chamber. The Stokes law referred to hereinabove is 1 R=61mrv wherein R is the resisting force, r is the droplet radius and v is the relative velocity of the sphere and the fluid. In this latter connection,

where v is the velocity of free fall of small heavy sphere, r is the radius, P is the density of material under investigation, and n is the viscosity of liquid.

Chamber I2 is much larger in diameter than column 10 and is open at the top to the ambient atmosphere. Consequently, the velocity of the fog in the upper direction in chamber 12 is less than that in column 10. Consequently, particles or droplets smaller than a size determined by the Stokes law for chamber 12 are carried off through the top of chamber 12 while droplets which are a cutout of the velocity distribution curve (FIG. 3) settle down onto the surface of an object 26 resting on a pedestal 28 which is provided in the bottom of chamber 12.

In the operation of the device shown in FIGS. 1 and IA, by controlling the size of the droplets, i.e., through the selection of the size of the jets 20 and 24 and by controlling the density of the fog in a controlled volume, i.e., the velocity of the fog through the selection of the volume of inert gas introduced into column 10, there can be built up a very uniform coating of photographic emulsion on the surface of object 26. The particle size and the density of the fog in a controlled volume lend themselves to continuous monitoring to assist in the building up of the coating of uniform thickness.

It is necessary to maintain the temperature of the system above the solidifying range of the photographic emulsion which is used, i.e., the solidifying range of gelatin. A suitable example of such range of temperatures is 40S0 C., thus temperature being maintained by a suitable heater 30.

To achieve optimum operation of the device shown in FIGS. 1 and 2, it is desirable that the inert gas stream, i.e., the driving stream be close to saturated by water vapor to avoid evaporation of water from the emulsion droplets in settling chamber 12. To achieve this affect, a film of water is caused to flow down the inner walls of chamber 12 by the introduction of hot water through pipe 32 and valve 33. The film is suitably achieved by introducing the water through a plate 35 having slanted perforations to cause the incoming water to be placed against the upper end of the inner walls of chamber ill-This film of water in addition to providing the desired high humidity atmosphere in chamber 12 produces the added advantage of producing continuous cleanliness by carrying off emulsion which might stick to the inner walls of chamber 12. Chamber 12 is provided with a suitable drain 34.

In further considering the structures contained in the device shown in FIGS. 1 and 1A, column 36 is a water jacket engirding column and chamber 12 which is maintained at the desired temperature by heater 30 to consequently maintain the whole system at such desired temperature, column 36 also being provided with a suitable drain. Inert gas is introduced through spray nozzle 24 via a water chamber 38 which is also maintained at the desired temperature by 30 whereby the gas is suitably warmed at the time of its entry into column 10. A removable baffle 40 is provided in settling chamber 12, baffle 40 functioning to initially receive on its surface the coarse drops resulting from splatter when the system is starting up. Pedestal 28 may suitably function as a cooling chamber and receives cooling water or inert gas through a pipe 29 entering thereinto, the cooling effect of chamber 28 effecting the solidifying of the liquid emulsion deposited on plate 26. Drain 34 and the hot water input through pipe 32 and valve 33 lend themselves to ready regulation whereby the level of water at the base of column 12 can be maintained at a fairly constant level.

With the introduction of the-hot water to provide both the high humidity atmosphere and to affect the cleaning of the inner walls of chamber 12, the device is to a large degree rendered self-cleaning and minimum contamination can be caused provided that clean gas is introduced through nozzle 24. In this connection, an important factor in the resulting cleanliness of the emulsion coating on object 26 is the fact that the emulsion is entirely airborne until it settles on object 26.

It has been mentioned above that chamber 12 is open at the top to the ambient atmosphere. In this connection, the cap 42 shown as closing the top of chamber 12 actually merely rests thereon with openings provided to enable free fluid flow out of the chamber. Cap 42 exerts no pressure on the interior of chamber 12. It is preferred that the structures in the device shown in FIGS. 1 and 1A be constructed of an inert relatively significant nonreactive material such as plastic. In the event that it is desired to monitor the process, the structures in the device could be constructed of a transparent plastic such as lucite. In this case, it is, of course, to be realized that the working light has to be selected in accordance with the sensitivity 7 of the emulsion to avoid its exposure.

Reference is now made to FIG. 2 wherein there is shown the particle size distribution chart resulting from the operation of the method according to the invention. That portion of the chart legended fine particles" are those particles or droplets in settling chamber 12 which are smaller than a size determined by the Stokes law for chamber 12. These are the droplets which are carried off through the top of chamber 12. The center vertical portion of the chart shown in FIG. 2 are the droplets of desired size which settle onto plate 26. The section of the chart legended coarse particles are those particles or drops which are elutriated, i.e., these are the drops which fall back in column 10, the control, of course, being efi'ected by the controlling of the volume of inert gas input through nozzle 24.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is: l. A method of coating a surface with a photographic emulsion comprising the steps of:

positioning in and at the base of a first larger vertical open chamber which communicates with a second smaller and substantially enclosed chamber at the upper end of said second chamber, an object having said surface to be coated;

introducing into said second chamber liquid ;r0ple ts of said emulsion under gas pressure while controlling the volume of said gas whereby, by elutriation, droplets of said emulsion exceeding a given size fall to the bottom of said second chamber and droplets sufficiently small according to Stokes law for said second chamber enter into said first chamber, those droplets of the droplets entering said first chamber and small enough according to Stokes law for said first chamber leaving said first chamber, those droplets of the droplets entering the first chamber and large enough to descend according to Stokes law for said first chamber settling down on said surface;

and permitting said immediately preceding step to continue until a desired thickness of a coating of said emulsion is deposited on said surface.

2. A method of coating as defined in claim 1 and further including the step of maintaining said system at a temperat$re at least exceeding the solidifying range of said emulsion.

3. A method of coating as defined in claim 2 wherein there is utilized as said gas which provides said gas pressure, one which is substantially saturated with water vapor.

4. A method of coating as defined in claim 3 and including the further step of flowing a film of water down the inner walls of said first chamber to substantially saturate said gas with water vapor.

5. A method of coating as defined in claim 4 and further including the step of controlling the temperature gradient at said surface whereby the emulsion droplets settling thereon are permitted to coalesce and to flow sufficiently to convert the droplets to a film. 

2. A method of coating as defined in claim 1 and further including the step of maintaining said system at a temperature at least exceeding the solidifying range of said emulsion.
 3. A method of coating as defined in claim 2 wherein there is utilized as said gas which provides said gas pressure, one which is substantially saturated with water vapor.
 4. A method of coating as defined in claim 3 and including the further step of flowing a film of water down the inner walls of said first chamber to substantially saturate said gas with water vapor.
 5. A method of coating as defined in claim 4 and further including the step of controlling the temperature gradient at said surface whereby the emulsion droplets settling thereon are permitted to coalesce and to flow sufficiently to convert the droplets to a film. 